Total hip replacement surgery requires accurate positioning of the acetabular and femoral components during the actual surgery. The acetabular component, sometimes referred to as the "socket" is aligned to the pelvis three dimensionally. Surgeons tend to orient the socket in terms of two dimensions. These two dimensions are termed anteversion, and inclination. Anteversion is the forward tilt of the axis of the hemispherical socket. Inclination is the upward tilt of the component. FIG. 1 depicts the inclination angle A in a front view of the pelvis. FIG. 2 depicts the anteversion angle B in a bottom view of the pelvis.
In anatomic terms, the anteversion is forward towards the front of the patient in the transverse plane. Inclination is in an upward direction in the coronal plane.
These two positions, anteversion and inclination can be critical to the stability of the total hip replacement. If the socket is placed either under or over certain angles, the hip will be unstable, and will dislocate. Dislocation usually requires an emergency transfer of the patient to a medical facility where anesthetics are administered and the hip manipulated back into place. Generally, the hip can be reduced or correctly positioned by closed means. However, in some cases, the patient may require an emergency operation to openly reduce or correctly position the hip joint. The degree of desired anteversion and inclination varies between surgeons, and there is no absolute value that is universally agreed upon. Clinical studies of dislocated prostheses show a significant extreme in the angular placement of the socket. In fact, the most common cause of dislocation that is due to component malposition is a malpositioned socket. Either under anteversion, retroversion (the plane of the socket is less than zero), or occasionally over anteversion can all lead to unstable replacements. If the inclination is above 60 degrees from the horizontal axis, there is a statistically increased risk of dislocation. Most surgeons report the ideal position of a socket at between 15 and 20 degrees of anteversion, and 40 to 45 degrees of inclination. When surgeons err, they prefer to place the socket with more anteversion, rather than less, and less inclination rather than more.
Anteversion is easier to estimate than inclination since there are more landmarks to reference. In the lower portion of the acetabulum, there is a ligament termed the transverse acetabular ligament. It has been described as a reference that can be used to determine anteversion. The surgeon more commonly estimates anteversion simply by referencing the coronal plane of the patient. Since the torso, shoulder, and pubis are generally visible or palpable, these landmarks can be utilized in the estimation of the coronal plane, and therefore the position of the socket in anteversion.
Inclination is much more difficult to assess than anteversion. The surgeon has few, if any, landmarks to spatially orient the pelvis in the sagittal plane. To rely on the visible bony anatomy of the acetabulum is misleading. The majority of acetabular are inclined steeper than 45 degrees. Attempting to use the rim of the acetabulum to orient inclination varies with the size of the acetabular socket. In addition, the pelvis is known to move in different directions during the operation. This makes it extremely difficult, if not impossible, to reference the pelvis during the insertion of the socket. Sophisticated electronic methods have been devised to track the position of the pelvis during surgery, but then can be expensive, invasive, and impractical for everyday use. Surgeons are left with a method of estimation of pelvis position to the horizontal plane. Manufacturers have provided guides that attempt to orient the socket to the horizontal plane. Some have even incorporated leveling devices to help orient the guide. However, all of these methods rely on the pelvis being oriented to that plane, a method that involves a crude estimation. lntraoperative radiographs have been described to confirm socket position, but have not been popular because of the impracticality of their use.